perlcc,
the byteloader and the supporting modules (B::C,
B::CC,
B::Bytecode,
etc.) are no longer distributed with the perl sources.
Those experimental tools have never worked reliably,
and,
due to the lack of volunteers to keep them in line with the perl interpreter developments,
it was decided to remove them instead of shipping a broken version of those.
The last version of those modules can be found with perl 5.9.4.

However the B compiler framework stays supported in the perl core,
as with the more useful modules it has permitted (among others,
B::Deparse and B::Concise).

It is now possible to write recursive patterns without using the (??{}) construct.
This new way is more efficient,
and in many cases easier to read.

Each capturing parenthesis can now be treated as an independent pattern that can be entered by using the (?PARNO) syntax (PARNO standing for "parenthesis number").
For example,
the following pattern will match nested balanced angle brackets:

/
^ # start of line
( # start capture buffer 1
< # match an opening angle bracket
(?: # match one of:
(?> # don't backtrack over the inside of this group
[^<>]+ # one or more non angle brackets
) # end non backtracking group
| # ... or ...
(?1) # recurse to bracket 1 and try it again
)* # 0 or more times.
> # match a closing angle bracket
) # end capture buffer one
$ # end of line
/x

Note, users experienced with PCRE will find that the Perl implementation of this feature differs from the PCRE one in that it is possible to backtrack into a recursed pattern, whereas in PCRE the recursion is atomic or "possessive" in nature. (Yves Orton)

It is now possible to name capturing parenthesis in a pattern and refer to the captured contents by name. The naming syntax is (?<NAME>....). It's possible to backreference to a named buffer with the \k<NAME> syntax. In code, the new magical hashes %+ and %- can be used to access the contents of the capture buffers.

Thus, to replace all doubled chars, one could write

s/(?<letter>.)\k<letter>/$+{letter}/g

Only buffers with defined contents will be "visible" in the %+ hash, so it's possible to do something like

The %- hash is a bit more complete, since it will contain array refs holding values from all capture buffers similarly named, if there should be many of them.

%+ and %- are implemented as tied hashes through the new module Tie::Hash::NamedCapture.

Users exposed to the .NET regex engine will find that the perl implementation differs in that the numerical ordering of the buffers is sequential, and not "unnamed first, then named". Thus in the pattern

/(A)(?<B>B)(C)(?<D>D)/

$1 will be 'A', $2 will be 'B', $3 will be 'C' and $4 will be 'D' and not $1 is 'A', $2 is 'C' and $3 is 'B' and $4 is 'D' that a .NET programmer would expect. This is considered a feature. :-) (Yves Orton)

Perl now supports the "possessive quantifier" syntax of the "atomic match" pattern. Basically a possessive quantifier matches as much as it can and never gives any back. Thus it can be used to control backtracking. The syntax is similar to non-greedy matching, except instead of using a '?' as the modifier the '+' is used. Thus ?+, *+, ++, {min,max}+ are now legal quantifiers. (Yves Orton)

The regex engine now supports a number of special-purpose backtrack control verbs: (*THEN), (*PRUNE), (*MARK), (*SKIP), (*COMMIT), (*FAIL) and (*ACCEPT). See perlre for their descriptions. (Yves Orton)

A new syntax \g{N} or \gN where "N" is a decimal integer allows a safer form of back-reference notation as well as allowing relative backreferences. This should make it easier to generate and embed patterns that contain backreferences. See "Capture buffers" in perlre. (Yves Orton)

The functionality of Jeff Pinyan's module Regexp::Keep has been added to the core. You can now use in regular expressions the special escape \K as a way to do something like floating length positive lookbehind. It is also useful in substitutions like:

A new prototype character has been added. _ is equivalent to $ (it denotes a scalar), but defaults to $_ if the corresponding argument isn't supplied. Due to the optional nature of the argument, you can only use it at the end of a prototype, or before a semicolon.

This has a small incompatible consequence: the prototype() function has been adjusted to return _ for some built-ins in appropriate cases (for example, prototype('CORE::rmdir')). (Rafael)

UNITCHECK, a new special code block has been introduced, in addition to BEGIN, CHECK, INIT and END.

CHECK and INIT blocks, while useful for some specialized purposes, are always executed at the transition between the compilation and the execution of the main program, and thus are useless whenever code is loaded at runtime. On the other hand, UNITCHECK blocks are executed just after the unit which defined them has been compiled. See perlmod for more information. (Alex Gough)

The built-in function readpipe() is now overridable. Overriding it permits also to override its operator counterpart, qx// (a.k.a. ``). Moreover, it now defaults to $_ if no argument is provided. (Rafael)

A new pragma, mro (for Method Resolution Order) has been added. It permits to switch, on a per-class basis, the algorithm that perl uses to find inherited methods in case of a multiple inheritance hierarchy. The default MRO hasn't changed (DFS, for Depth First Search). Another MRO is available: the C3 algorithm. See mro for more information. (Brandon Black)

Note that, due to changes in the implementation of class hierarchy search, code that used to undef the *ISA glob will most probably break. Anyway, undef'ing *ISA had the side-effect of removing the magic on the @ISA array and should not have been done in the first place.

Upon import, using lib => 'Foo' now warns if the low-level library cannot be found. To suppress the warning, you can use try => 'Foo' instead. To convert the warning into a die, use only => 'Foo' instead.

In addition, the default math-backend (Calc (Perl) and FastCalc (XS)) now support storing numbers in parts with 9 digits instead of 7 on Perls with either 64bit integer or long double support. This means math operations scale better and are thus faster for really big numbers.

Locale::Maketext::Simple, needed by CPANPLUS, is a simple wrapper around Locale::Maketext::Lexicon. Note that Locale::Maketext::Lexicon isn't included in the perl core; the behaviour of Locale::Maketext::Simple gracefully degrades when the later isn't present.

Params::Check implements a generic input parsing/checking mechanism. It is used by CPANPLUS.

The assertions pragma, its submodules assertions::activate and assertions::compat and the -A command-line switch have been removed. The interface was not judged mature enough for inclusion in a stable release.

The warnings pragma doesn't load Carp anymore. That means that code that used Carp routines without having loaded it at compile time might need to be adjusted; typically, the following (faulty) code won't work anymore, and will require parentheses to be added after the function name:

less now does something useful (or at least it tries to). In fact, it has been turned into a lexical pragma. So, in your modules, you can now test whether your users have requested to use less CPU, or less memory, less magic, or maybe even less fat. See less for more. (Joshua ben Jore)

It's now possible to access the lexical pragma hints (%^H) by using the method B::COP::hints_hash(). It returns a B::RHE object, which in turn can be used to get a hash reference via the method B::RHE::HASH(). (Joshua ben Jore)

As the old 5005thread threading model has been removed, in favor of the ithreads scheme, the Thread module is now a compatibility wrapper, to be used in old code only. It has been removed from the default list of dynamic extensions.

PerlIO::scalar will now prevent writing to read-only scalars. Moreover, seek() is now supported with PerlIO::scalar-based filehandles, the underlying string being zero-filled as needed. (Rafael, Jarkko Hietaniemi)

study() never worked for UTF-8 strings, but could lead to false results. It's now a no-op on UTF-8 data. (Yves Orton)

The signals SIGILL, SIGBUS and SIGSEGV are now always delivered in an "unsafe" manner (contrary to other signals, that are deferred until the perl interpreter reaches a reasonably stable state; see "Deferred Signals (Safe Signals)" in perlipc). (Rafael)

When a module or a file is loaded through an @INC-hook, and when this hook has set a filename entry in %INC, __FILE__ is now set for this module accordingly to the contents of that %INC entry. (Rafael)

The -w and -t switches can now be used together without messing up what categories of warnings are activated or not. (Rafael)

Duping a filehandle which has the :utf8 PerlIO layer set will now properly carry that layer on the duped filehandle. (Rafael)

Localizing an hash element whose key was given as a variable didn't work correctly if the variable was changed while the local() was in effect (as in local $h{$x}; ++$x). (Bo Lindbergh)

The anonymous hash and array constructors now take 1 op in the optree instead of 3, now that pp_anonhash and pp_anonlist return a reference to an hash/array when the op is flagged with OPf_SPECIAL (Nicholas Clark).

If you find what you think is a bug, you might check the articles recently posted to the comp.lang.perl.misc newsgroup and the perl bug database at http://rt.perl.org/rt3/ . There may also be information at http://www.perl.org/ , the Perl Home Page.

If you believe you have an unreported bug, please run the perlbug program included with your release. Be sure to trim your bug down to a tiny but sufficient test case. Your bug report, along with the output of perl -V, will be sent off to perlbug@perl.org to be analysed by the Perl porting team.